Another puzzle for summertime. And again, I won't propose my explanation immediately, just in case it's not watertight...

All of you have probably noticed that distant sounds can be heard better if you're leeward (hope it's the right English word for "downstream in the wind"). This effect is especially noticeable with distant sounds, like a train passing a couple of km away.

At first thought, it looks natural: sound carried by moving air has less distance to travel leeward. But at second thought, this explanation doesn't hold: with a good wind of 10m/s (already too noisy by itself), the air distance reduces by 3%, aiding the sound pressure by only 0.25dB - far too little, as we need 10dB to perceive a significant difference, and rather 20dB change to notice something we haven't been made aware of.

So we need a better explanation...Enjoy!

bm1957

20th July 2008 - 04:21 PM

I think it's something to do with the bending of sound waves which focuses them at distant points from the source. Something to do with something like that anyway!

Anyone got a less vague idea?

Sinister Utopia

20th July 2008 - 04:49 PM

QUOTE (bm1957+)

Anyone got a less vague idea?

Yes!Does it have anything to do with those little hairs in your ear?

barakn

20th July 2008 - 07:33 PM

Sounds are generally emitted from objects. These objects are in the wind and the air flowing around them becomes turbulent on the downwind side. As the wind carries this turbulent air further from the object the size of the affected air stream spreads outward. The object could thus be seen as the vertex of a cone of disrupted air, and perhaps the sound is focused by this cone of turbulent air like a megaphone.

kjw

20th July 2008 - 09:02 PM

the medium in which the sound is propagating is in motion, relative to your ear. the sound reaches you faster since it is traveling the speed of sound + speed of medium, by reaching you faster it is capable of reaching further distances from the source. the degeneration rate of sound does not change, but the distance the degeneration is spread over is increased.

barakn

20th July 2008 - 09:32 PM

QUOTE (kjw+Jul 20 2008, 03:02 PM)

the medium in which the sound is propagating is in motion, relative to your ear. the sound reaches you faster since it is traveling the speed of sound + speed of medium, by reaching you faster it is capable of reaching further distances from the source. the degeneration rate of sound does not change, but the distance the degeneration is spread over is increased.

Enthalpy discounted that in the OP, if you care to read it a little more carefully.

kjw

21st July 2008 - 02:42 AM

QUOTE

barakn Posted on Today at 7:32 AM Enthalpy discounted that in the OP, if you care to read it a little more carefully

Another puzzle for summertime. And again, I won't propose my explanation immediately, just in case it's not watertight...

All of you have probably noticed that distant sounds can be heard better if you're leeward (hope it's the right English word for "downstream in the wind"). This effect is especially noticeable with distant sounds, like a train passing a couple of km away.

At first thought, it looks natural: sound carried by moving air has less distance to travel leeward. But at second thought, this explanation doesn't hold: with a good wind of 10m/s (already too noisy by itself), the air distance reduces by 3%, aiding the sound pressure by only 0.25dB - far too little, as we need 10dB to perceive a significant difference, and rather 20dB change to notice something we haven't been made aware of.

So we need a better explanation...Enjoy!

Perhaps is something similar to a flowing fluid through a pipe.

If you have water flowing through a pipe, the molecules most in contact with the pipe will move slower, while molecules near the center will move faster. Molecules that, in the open, would spread out, instead bounce off of the inside surface of the pipe, back into the stream.

Now if we visualize the moving air in this fashion, as well as the propagating sound waves, it is possible that the sound waves hit a boundary with the non-moving air (or slower moving air) and are reflected back into the moving air, thus causing the sound waves to remain concentrated instead of spreading, This could act as a "megaphone", channeling a larger percentage of the original sound energy in the direction of the listener, not just getting it there faster.

I believe the wind shear is indeed the right answer. Nice page at the link, by the way.

That is: wind is stronger at altitude than near to the ground, and increases or decreases the speed of sound. So in leeward propagation, the resulting speed is smaller near to the ground, and this guides soundwaves near to the ground just as a fibre guides light. Windward, sound is dispersed towards the sky.

Well done everybody!

excaza

21st July 2008 - 07:11 PM

Sound travels differently at higher altitudes because the medium is less dense, not (only) because of the winds.

kjw

21st July 2008 - 09:28 PM

QUOTE

bm1957 Posted on Yesterday at 4:00 PM Ooh, I like that explanation; it means that I might have been close!

well done

as for my lack of attention to information in front of my nose... not at all well done

Sapo

21st July 2008 - 09:54 PM

All that have replied to this thread so far would be very welcome at my place!

This is the sort of loose, but reasoned and polite discussion that could blow the 'tumbleweeds' out of the Joint There is so much humor in such a talk, as well as good acoustical engineering...

PF is wonderful, but they don't have recipes.

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